Superconductivity and Phase Stability of Potassium-Doped Biphenyl† Cite This: Phys

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Superconductivity and phase stability of potassium-doped biphenyl† Cite this: Phys. Chem. Chem. Phys., 2018, 20,25217 Guo-Hua Zhong, a Dong-Yu Yang,b Kai Zhang,b Ren-Shu Wang,b Chao Zhang,c Hai-Qing Lin*d and Xiao-Jia Chen*b

Phenyl molecules are proposed as potential high-temperature superconductors due to exhibiting interesting properties. Here, we report the discovery of superconductivity with the critical temperature

(Tc)ofB7.2 Kelvin in potassium (K)-doped biphenyl (C12H10). The dc magnetic susceptibility measurements

provide solid evidence for the presence of the Meissner eﬀect in KxC12H10. The Raman spectra detected Received 14th August 2018, bipolaronic characteristics in this superconducting state, which are proposed to account for the electron Accepted 6th September 2018 pairing. Theoretical simulations provided the information of the crystal structure of KxC12H10. Combining

DOI: 10.1039/c8cp05184d XRD data with formation energy, we suggest that the superconducting phase corresponds to K2C12H10 or with a small charge fluctuation in a layered structure. The discovery of superconductivity in K-doped rsc.li/pccp biphenyl vastly expands the potential applications in the superconducting field.

8 1 Introduction under 1.2 GPa. Then, in 2003, the Tc was raised to 13.4 K in the BEDT-TTF salt family under 8.2 GPa.9 Recently, an inter- Organic materials have attracted increasing interest because of esting superconductivity has been discovered in polycyclic their many fascinating phenomena such as low dimensionality, aromatic hydrocarbons (PAHs) doped by alkali metals.10–15

strong electron–electron and electron–phonon interactions and The PAH materials exhibit the characteristics of multi-Tc

the proximity of antiferromagnetism, insulator states and phases and the highest value of Tc increases with the number superconductivity. Especially, organic based compounds were of benzene rings, reaching 33 K in K-doped 1,2:8,9-dibenzo- suggested as candidates for high temperature or room tempera- pentacene.13 In addition, researchers observed that the poly- ture superconductors since the electrons can interact with much (para-phenylene) (PPP) materials, such as K-doped p-terphenyl,16–20 higher excitation energy than the phonon energy in these p-quaterphenyl,21,22 and p-quinquephenyl,23 which are non-edge-

Published on 07 September 2018. Downloaded by Trinity College Dublin 10/12/2018 6:12:16 AM. materials.1,2 Numerous studies have been done over a half shared aromatic hydrocarbons, also possess rich superconduc-

century. In 1965, the first carbon-based superconductor C8A tivities. It is worth mentioning that p-terphenyl was found to be 19 (A represents the alkali metal) was studied, exhibiting a super- superconducting with Tc as high as 123 K, which is comparable 3 conducting transition temperature (Tc) of 0.02–0.55 K. In 2005, to the record high Tc of about 130 K in some cuprate super- 24 the Tc values of graphite-like superconductors were raised to conductors at ambient pressure. This discovery of high Tc 4,5 6.5 and 11.5 K for YbC6 and CaC6, respectively. In 1991, superconductivity in p-terphenyl seemingly supports the above superconductivity was also observed in the complete carbon idea suggested by Little and Ginzburg. B 6 material of fullerene by doping potassium (K), with Tc 18 K. Interestingly, the low-Tc phase with Tc in the range of 5–7 K 7 10–15 Furthermore, the Tc in cesium-doped fullerene reached to 38 K. exists in observed PAH superconductors, though some

In 1980, a real organic superconductor consisting of C and H PAHs exhibit the characteristics of multi-Tc phases. By explor- atoms, di-(tetramethyltetraselenafulvalene)-hexauorophosphate, ing the superconductivity of K-doped solid benzene, which is a

named (TMTSF)2PF6, was reported with Tc equaling 0.9 K basic unit of aromatic compounds (the electron–phonon cou-

pling constant l = 0.67 and Tc = 6.2 K), theoretical studies

a Shenzhen Institutes of Advanced Technology, Chinese Academy of Sciences, pointed out that the low Tc phase of 5–7 K is a common feature Shenzhen 518055, China for all aromatic hydrocarbon superconductors and is easy to 25 b Center for High Pressure Science and Technology Advanced Research, obtain due to better stability. The subsequent experimental Shanghai 201203, China. E-mail: [email protected] investigations confirmed the observation, for example, 7.2 K in c Department of Physics, Yantai University, Yantai, 264005, China 17 22 23 d p-terphenyl and p-quaterphenyl, 7.3 K in p-quinquephenyl, Beijing Computational Science Research Center, Beijing, 100193, China. 0 26 27 E-mail: [email protected] 7.2 K in 2,2 -bipyridine, and 3.5/7.2 K in triphenylbismuth. † Electronic supplementary information (ESI) available: Atomic coordinates and The p electron network plays an important role in the super- bond lengths for pristine and doped biphenyl. See DOI: 10.1039/c8cp05184d conductivity of PAH and PPP compounds and drives a strong

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couplingofthechargecarrierstothelattices.Theelectron– 2 Experimental and phonon interactions can account for the superconductivity of computational methods this unified superconducting phase.25,28 Within the frame- work of BCS theory, an increase of doping concentration may 2.1 Experimental details result in a stronger electron–phonon interaction, which is High quality potassium and crystalline biphenyl were pur- 29 helpful for obtaining a higher Tc, and the pressure can also chased from Sinopharm Chemical Reagent and Alfa Aesar, 30 drive a higher Tc. respectively. In our experiments, the doped biphenyl was However, the electron–phonon interaction is not enough synthetized by heating a mixture of small pieces of fresh

to produce the high Tc superconductivity such as 18 K potassium and biphenyl with a mole ratio of 3 : 1 at a tempera- in picene,10 15 K in coronene,11 33 K in 1,2:8,9-dibenzo- ture of 1201–130 1C for 48 hours. All the synthesis procedures pentacene,13 and 43/123 K in p-terphenyl.18 Other factors were performed in a glove box with moisture and oxygen levels

mayplayimportantrolesinthehighTc superconductivity. less than 0.1 ppm, and the sample was sealed in a quartz tube Keyfeaturesoforganicsuperconductorsarethelowdimen- under a high vacuum of better than 1 104 Pa in the heating sionality of the materials and strong electron–electron corre- process. The dc magnetization measurements were performed

lation. The fulleride Cs3C60 was found to be a true Mott in the temperature range from 1.8 K to 300 K by a commercial insulator because of strong electronic correlation effects,31 Magnetic Property Measurement System (Quantum Design). In though it could be turned into a superconductor by pressure.7 the Raman scattering experiments, a 660 nm excitation was

Superconductivity induced by pressure in Cs3C60 was found applied to obtain the Raman spectra of the pristine and doped 32 to be related to the antiferromagnetic Mott insulator. K3 biphenyl. The laser power was selected as less than 2 mW before picene was also revealed as a strongly correlated electron a 20 objective to avoid possible damage of samples. The X-ray system,33,34 which results in the absence of metallicity.35 diffraction experiments were performed in the 2-theta range of Theoretical calculations predicted that K-doped PAHs in their 51–601 by an X-ray diffractometer (Empyrean, PANalytical B.V.). ground state exhibit near antiferromagnetic behaviour.34,36,37 The samples were put on specific glasses and then covered with Especially for K-doped p-terphenyl, the bipolaronic thin films to isolate the air. characteristic has been identified from Raman scattering 17–19 measurements. Photoemission spectroscopy conducted 2.2 Computational details on surface K-doped p-terphenyl crystals showed a gap persist- ing up to at least 120 K that shares a similar temperature To study the structural and electronic properties of pristine and K-doped biphenyl, we employed the Vienna ab initio simulation dependence to the obtained spectra of Bi–Sr–Ca–Cu–O 41,42 superconductors.38 Furthermore, an analogous gap below package (VASP) based on the projector augmented wave 50 K was also found in K-doped p-terphenyl films fabricated method. For the planewave basis-set expansion, an energy cut- 39 oﬀ of 600 eV was adopted. The Monkhorst–Pack k-point grids on Au(111). All these features indicate that the high Tc superconductivity of organic materials with p-electron net- were generated according to the specified k-point separation of 0.02 Å1 and the convergence thresholds were set as 106 eV in works cannot simply be explained by the electron–phonon 3 1 mechanism. This indicates that both the electron–phonon energy and 10 eV Å in force. The generalized gradient form

Published on 07 September 2018. Downloaded by Trinity College Dublin 10/12/2018 6:12:16 AM. interaction and electron–electron correlations work together (GGA) of the exchange–correlation functional (Perdew–Burke– Ernzerh of 96, PBE) was adopted.43 Considering the non-local to enhance the Tc of aromatic hydrocarbons with increasings 25 interactions, we added the correction of van der Waals (vdW) numbers of benzene rings. 44 Due to the appearance of superconductivity and possible in the version of vdW-DF2 in the calculations. The necessity of the vdW-DF2 functional has been confirmed by our previous high Tc, PPP superconductors provide a good system to study 29,45 the interplay of strong electron–electron and electron–phonon studies. interactions in a low-dimensional system. Comparing with

p-terphenyl, biphenyl (C12H10) is the shortest polymer with only two phenyl rings connected by a single C–C bond. 3 Results and discussion It shows a relatively simple monoclinic structure, and it undergoes an incommensurate triclinic transition at 3.1 Meissner eﬀect about 40 K.40 In this work, therefore, we have synthesizedK- The Meissner eﬀect and zero resistivity are two fundamental

doped biphenyl (KxC12H10) and observed the Meissner features of superconductivity. Actually, metal-doped carbon- effect in this material. The dc magnetic measurements based superconductors were mainly detected from magnetic 10–15 support the existence of superconductivity with Tc B 7.2 K measurement. However, with its small diamagnetic volume in this material. Combining with the first-principles calcula- and easily damaged crystal structure, the metallic state of

tions, we predicted the crystal structure and electronic band KxC12H10 was hard to realize even below Tc. Difficulties in feature of this superconducting phase to understand the detecting weak superconductivity from resistivity measure- superconductivity of K-doped biphenyl. Our results report ments can be overcome by using magnetic susceptibility experi-

the smallest p-oligophenyl superconductor and present the ments. Hence, the superconductivity of KxC12H10 was mainly possible microstructure. characterized by magnetization measurements.

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Fig. 1 (a) The temperature dependence of dc magnetic susceptibility w for Fig. 2 (a) The magnetic field dependence of magnetization for K biphe- K biphenyl in the applied magnetic field of 20 Oe with field-cooling (FC) x x nyl at various temperatures in the superconducting state. (b) The magne- and zero-field cooling (ZFC). Inset is the black synthetic sample of K x tization hysteresis loop with scanning magnetic field along two opposite biphenyl. (b) The temperature dependence of w for K biphenyl measured x directions up to 800 Oe measured at various temperatures in the super- at various magnetic fields up to 600 Oe in the ZFC run. conducting state.

Fig. 1(a) shows the dc magnetic susceptibility w of the powder Published on 07 September 2018. Downloaded by Trinity College Dublin 10/12/2018 6:12:16 AM. sample KxC12H10 [inset of Fig. 1(a)] versus temperature measured The magnetic field dependence of the magnetization for the in the zero-field cooling (ZFC) and field cooling (FC) cycles at superconducting KxC12H10 at various temperatures from 2 K 20 Oe. We can see a sudden drop of w at the temperature of 7.2 K to 6.5 K with low applied fields is summarized in Fig. 2(a). in the ZFC run, indicating that a superconducting transition The linear behavior of the magnetic-dependent magnetization

occurs in this sample KxC12H10. The superconducting transition signals the Meissner eﬀect in this superconductor. The super-

temperature Tc was determined as 7.2 K. Supposing that the conductivity is suppressed as the temperature increases above density r of this sample is about 3 g cm3, we can obtain the 7 K. Fig. 2(b) shows the magnetization hysteresis loop with shielding fraction 4pwr = 0.004% from Fig. 1(a). Such a small magnetic field along two opposite directions up to 800 Oe at shielding fraction is due to the existence of impurities and the various temperatures from 2 K to 6.5 K in this superconducting smaller size of crystallites compared to the London penetration state. The symmetric hysteresis loops indicate that this sample depthinthispowdersample.Noticeably,bothZFCandFCw show exhibits a strong bulk pinning, which is certification of a type II

an upturn trend at a lower temperature than Tc,whichimpliesa superconductor. strong paramagnetic signal. The magnetic susceptibility of 3.2 Raman spectra KxC12H10 as a function of temperature in the ZFC run under diﬀerent magnetic fields was alsoobtainedtodemonstratethe To understand this superconducting material from the aspect

superconducting transition of KxC12H10.AsshowninFig.1(b),the of the molecular dynamics process, we measured the Raman

diamagnetism of KxC12H10 gradually decreases with increasing scattering spectra of the pristine and the doped biphenyl by magnetic field. That is, the superconducting transition gradually using a laser wavelength of 660 nm, respectively shown in Fig. 3. disappears with increasing magnetic field since it is suppressed According to previous studies,26,46,47 all of the major peaks in the by the applied magnetic field. However, the superconductivity can spectra of pristine biphenyl can be assigned and classified as the be observed until the magnetic field is larger than 500 Oe. ring deformation, ring breathing, C–H bending, and C–C

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Table 1 Optimized crystal parameters of C12H10 and KxC12H10, including the space-group, the lattice constants, the angle, and the volume of unit cell

System Space-group a (Å) b (Å) c (Å) a (1) b (1) g (1) V (Å3)

50 C12H10 P21/c 11.763 5.580 7.820 90 126.9 90 410.6 C12H10 P21/c 11.862 5.532 7.965 90 127.6 90 414.0 K1C12H10 P21 7.193 7.142 9.411 90 104.0 90 469.1 K1.5C12H10 P1 9.218 8.792 12.158 88.0 148.4 92.4 516.4 K2C12H10 P21/c 11.445 5.473 10.468 90 123.0 90 550.1 K3C12H10 P21/c 12.755 6.099 9.032 90 102.5 90 686.1

to predict the crystal and electronic structures of K-doped biphenyl. First, we performed a test in the case of the pristine sample. As shown in Table 1, we obtained the optimized crystal parameters of a =11.862Å,b =5.580Å,c =7.965Å,b =127.61, and V =414.0Å3, which are in good agreement with the Fig. 3 Raman spectra of the doped (upper) and pristine (bottom) biphenyl 51 excited by a laser wavelength of 660 nm. experimental values. The result implies the feasibility of the theoretical study. Then, we considered four possible doping

concentrations, x =1,1.5,2,and3inKxC12H10. Selecting the stretching. The peaks in the range of 200–800 cm1 mainly structures with the lowest total energy for each doping level, the come from the the ring deformation. The bands from 800 to optimized crystal parameters are summarized in Table 1 and 1000 cm1 and from 1000 to 1250 cm1 were formed by the C–H the geometrical configurations are shown in Fig. 4. Detailed bending out-plane and the C–H bending in-plane, respectively. atomic coordinates and bond lengths for pristine and K-doped The two peaks near 1000 and 1250 cm1 arise from the ring biphenyl stable structuresarepresentedintheESI† (Tables S1–S5 breathing and the inter-ring C–C stretching, respectively. and Fig. S1–S5). After doping K metal into biphenyl, great changes The peaks around 1600 cm1 are due to the intra-ring C–C tookplaceinthelatticeconstants,andthevolumeoftheunitcell stretching. gradually increased with the doping content. As shown in It is well-known that polarons and bipolarons are able to Fig. 4, the biphenyl molecules respectively form the herring- form in conducting polymers.48 Moreover, bipolaronic charac- bone configuration in K1C12H10,K1.5C12H10, and K3C12H10 10–15 terization in alkali-metal doped PPPs synthesized by diﬀerent similar to K-doped PAHs, however, a typical layered struc- methods has been extensively studied in previous works.49,50 ture is observed in K2C12H10. Noticeably, there is a tilted angle The almost separated two intra-ring C–C stretching modes at between two successive benzene rings in K1C12H10, but this around 1595 cm1 in the spectra of pristine biphenyl downshift tilted angle disappears when increasing the K content. Viewing and merge with bipolaronic bands localized at 1586 cm1. perpendicular to the molecular surface, the two ends of the The observation of the 1475 cm1 mode with no corresponding biphenyl molecule are favorable for K atoms.

Published on 07 September 2018. Downloaded by Trinity College Dublin 10/12/2018 6:12:16 AM. band in the parent sample can be considered as the fingerprint To examine the possible doping situation corresponding to for the formation of bipolarons. The 1325 cm1 and 1360 cm1 our experiment, we analyzed the thermodynamic stability of these considered doping cases by calculating the formation bands of KxC12H10 originate from the inter-ring C–C stretching mode at 1265 cm1 of the pristine sample. The upshifts in energy Ef. In this work, the Ef for the doping level x is defined as wavenumber reflect the decrease of length in the C–C bonds a function of K chemical potential with the fomula between rings. The 1260 cm1 mode present in the Raman

spectra of KxC12H10 is due to the C–H bending in-plane, corresponding to the 1200 cm1 mode in the pristine sample. The two bipolaronic bands at 1160 and 1200 cm1 are induced by the C–H bending in-plane, corresponding to the 1155 cm1 mode in the pristine sample. The bands localized at 965 cm1 are from the C–H bending out-plane. These bands also exhibit bipolaronic characteristics. As a result, the Raman character-

ization of KxC12H10 proves the existence of bipolarons, which possibly accounts for the observed superconductivity in

KxC12H10.

3.3 Crystal structure and stability Fig. 4 Optimized crystal structures of K C H viewed from diﬀerent The identification of crystal structure and the investigation of x 12 10 directions. (a–d) Correspond to K1C12H10,K1.5C12H10,K2C12H10,andK3C12H10, electronic properties are helpful for exploring this super- respectively. Red, gray, and green balls represent the C, H, and K atoms, conductor. Therefore, we carried out first-principles calculations respectively.

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Fig. 5 Calculated formation energy of KxC12H10 as a function of the K chemical potential.

bulk bulk Ef = Edoped Epristine xmK x[mK mK ] (1)

where Edoped and Epristine are the total energy of the doped and bulk host crystal, respectively, mK can be obtained from the energy Fig. 6 Comparison of X-ray diﬀraction patterns of the pristine and doped +0.1e per K atom in the K metal with the bcc structure, x is the doping biphenyl between experiments and theoretical calculations. (K2C12H10) 0.1e and (K2C12H10) imply the injection and removal of a small number of concentration, and mK is the chemical potential of the K bulk electrons for K2C12H10. species. Here mK = mK means that the element is so rich that

the pure element phase can form. Ef o 0 indicates that the doped compound can stably exist. From the calculated for- shifts towards a lower angle in K3C12H10. Thus, we can exclude mation energy shown in Fig. 5, all of the considered doped the possibility of K1C12H10,K1.5C12H10, and K3C12H10 from the phases are able to exist when the chemical potential of K XRD patterns. Especially, K1C12H10 exhibits antiferromagnetism satisfies a certain condition. Comparing several doping levels, 52 instead of superconductivity. Both K1.5C12H10 and K3C12H10 however, K2C12H10 is more stable since it has a lower formation possess a higher formation energy, as shown in Fig. 5. The energy than the other phases in a wide range of chemical structure of K2C12H10 produces XRD peaks near to those of the

Published on 07 September 2018. Downloaded by Trinity College Dublin 10/12/2018 6:12:16 AM. potential because of the Ef o 0 condition. This suggests that experimentally doped sample. As shown in Fig. 6, K2C12H10 with the experimentally observed superconducting phase of 7.2 K is +0.1e a small fluctuating charge such as (K2C12H10) (injected possibly K2C12H10. electron) exhibits coincident XRD peaks with the experimental sample, especially the peaks at low angles. Moreover, our 3.4 X-ray diffraction pattern previous studies demonstrated the stability and feasibility of The X-ray diffraction (XRD) patterns shown in Fig. 6 further doped PAHs with small charge.45 Therefore, we can conclude follow the superconducting state observed by experiment that the observed superconducting state corresponds to B corresponding to the doping level of x 2. Comparing the K2C12H10 or with a small fluctuated charge. The crystal struc- XRD spectra of the pristine and doped biphenyl from experi- ture of this superconducting material is shown in Fig. 4(c). ments, the (100) peak at 91 remains in K-doped biphenyl, which indicates that no big change exists in the lattice constant of 3.5 Electronic structures % a after doping. A new XRD peak marked as (102) appears at Fig. 7(a) shows the band structure of K2C12H10 along the high- about 161 for this experimentally doped sample that is different symmetry k-points G(0,0,0),B(0.5,0,0),A(0.5, 0.5, 0), Y(0,0.5,0), from the pristine biphenyl, which means that there is an Z(0,0,0.5),D(0.5, 0, 0.5), E(0.5, 0.5, 0.5), and C(0, 0.5, 0.5) in the expansion of lattice constants along the c direction. Based on Brillouin zone. Fig. 7(b) shows the total density of states (DOS) +0.1e these constraints, we first checked the XRD patterns of the andprojecteddensityofstates(PDOS)ofK2C12H10,(K2C12H10) 0.1e predicted structures of K1C12H10,K1.5C12H10,K2C12H10, and and (K2C12H10) . There is an indirect overlap between conduc-

K3C12H10 shown in Fig. 6. It was found that several new XRD tion bands and valence bands at the Fermi level of K2C12H10 peaks emerge in the range of 101–161 and the peak at 91 shifts showninFig.7(a),whichmeansasmallamountofelectronic towards to a higher angle in the two kinds of doping situations states [0.47 states per eV per f.u. shown in Fig. 7(b)] at the Fermi

of K1C12H10 and K1.5C12H10. Moreover, the peak at 91 greatly level. The density of states at the Fermi level is less than 4.6 states

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employing first-principles calculations. Combining XRD data with formation energy, we suggested that the superconducting

phase corresponds to K2C12H10 or with a small charge fluctuation in a layered structure. Electronic states demonstrate the B reasonability of the superconductivity of Tc 7 K. The discovery of the superconductivity in K-doped biphenyl is an interesting and significant achievement in organic materials research. This discovery vastly expands the potential applications of superconducting materials.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The work was supported by the National Natural Science Foundation of China (Grant No. 61574157 and 61774164) and the Basic Research Program of Shenzhen (Grant No. JCYJ20170818153404696 and JCYJ20150925163313898). We also acknowledge financial support from NSAF U1530401 and computational resources from the Beijing Computational Science Research Center. The calculation was partially supported by the Special Program for Applied Research on Super Computation of the NSFC-Guangdong Joint Fund (the second phase) under Grant No. U1501501. Experiments at HPSTAR were supported by the National Key R&D Program of China (Grant No. 2018YFA0305900).

Fig. 7 The band structure of K2C12H10 (a) and the DOS/PDOS of K2C12H10, +0.1e 0.1e (K2C12H10) and (K2C12H10) (b). Zero energy denotes the Fermi level. Notes and references

1 W. A. Little, Phys. Rev., 1964, 134, A1416–A1424. 25 per eV per f.u. of K2C6H6 where the Tc was predicted as 6.2 K. 2 V. L. Ginzburg, Sov. Phys. Usp., 1976, 19, 174–179.

Published on 07 September 2018. Downloaded by Trinity College Dublin 10/12/2018 6:12:16 AM. Thus, based on the electron–phonon coupling mechanism, the 3 N. B. Hannay, T. H. Geballe, B. T. Matthias, K. Andres, predicted superconducting transition temperature Tc of K2C12H10 P. Schmidt and D. MacNair, Phys. Rev. Lett., 1965, 14, is low. However, with a small fluctuated charge such as 225–226. +0.1e (K2C12H10) , the density of states at the Fermi level will increase 4 T. E. Weller, M. Ellerby, S. S. Saxena, R. P. Smith and to 3.4 states per eV per f.u. as shown in Fig. 7(b). There will be a N. T. Skipper, Nat. Phys., 2005, 1, 39–41. big value for more additional charge.Additionally,fromthePDOS 5 N. Emery, C. He´rold, M. d’Astuto, V. Garcia, C. Bellin, showninFig.7(b),weobservethat the electronic states near the J. F. Mareˆche´, P. Lagrange and G. Loupias, Phys. Rev. Lett., Fermi level are mainly contributed by the C 2p state, which means 2005, 95, 087003. that the C atoms dominate the superconductivity of K-doped 6 A. F. Hebard, M. J. Rosseinsky, R. C. Haddon, D. W. Murphy, biphenyl. S. H. Glarum, T. T. M. Palstra, A. R. Ramirez and A. R. Kortan, Nature, 1991, 350, 600–601. 4 Conclusions 7 A. Y. Ganin, Y. Takabayashi, Y. Z. Khimyak, S. Margadonna, A. Tamai, M. J. Rosseinsky and K. Prassides, Nat. Mater., In summary, we have synthesized K-doped biphenyl and carried 2008, 7, 367–371. out the magnetic susceptibility and Raman scattering measure- 8D.Je´rome, A. Mazaud, M. Ribault and K. Bechgaard, J. Phys. ments. Based on the Meissner eﬀect, we found that K-doped Lett., 1980, 41, L95–L98.

biphenyl is superconducting with a Tc of 7.2 K. From the 9 H. Taniguchi, M. Miyashita, K. Uchiyama, K. Satoh, N. Moˆri, Raman spectra, the bipolaronic characteristics could be H. Okamoto, K. Miyagawa, K. Kanoda, M. Hedo and observed in this superconducting sample, which are proposed Y. Uwatoko, J. Phys. Soc. Jpn., 2003, 72, 468–471. to account for the electron pairing. Furthermore, the crystal 10 R. Mitsuhashi, Y. Suzuki, Y. Yamanari, H. Mitamura, structure of this superconducting phase was suggested by T. Kambe, N. Ikeda, H. Okamoto, A. Fujiwara, M. Yamaji,

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